Trebuchet: Use preference provider to lock/unlock homescreen

mSharedPreferences uses "com.cyanogenmod.trebuchet.prefs", while the Preferences activity uses
"com.cyanogenmod.trebuchet_preferences". Use PreferenceProvider to sync lock/unlock homescreen.

Change-Id: Ib9312a9f8cffe3f009b8cd8d9b9789d459c1bd96
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>